privops: refactor initialisation/finalisation

Rename PRV_Initialise() to PRV_StartHelper() and add a new
initialisation function, which just sets the helper fd to -1. Move
the initialision/finalisation calls from the system drivers to main.c.
If privops is not included in the build, define empty macros for the
function names, so their calls don't have to be wrapped in #ifdefs.

privops.h

@@ -48,7 +48,12 @@ int PRV_BindSocket(int sock, struct sockaddr *address, socklen_t address_len);
 
 #ifdef PRIVOPS_HELPER
 void PRV_Initialise(void);
+void PRV_StartHelper(void);
 void PRV_Finalise(void);
+#else
+#define PRV_Initialise()
+#define PRV_StartHelper()
+#define PRV_Finalise()
 #endif
 
 #endif